1. Field of the Invention
This invention relates to a lens barrel for driving a lens holding member which holds a lens, by use of a cam structure. More particularly, the invention concerns a lens barrel of what can be called a tube collapsing type in which, for better portability in the non-photographic state, a lens holding member moves to protrude out of and sink into a barrel body portion along the optical axis direction.
2. Description of the Related Art
An example of such collapsing type lens barrel having a zoom function is disclosed in Japanese Laid-Open Patent Application No. 2002-267917. This lens barrel is provided with a cam follower which protrudes from the outer peripheral surface of a lens holding frame for holding a movable lens, a cam slot which is formed on the inner peripheral surface of a cam tube and with which the cam follower engages, and a guide tube for interrupting the rotation of the lens holding frame to guide the lens holding frame rectilinearly.
Furthermore, the cam tube is rotationally moved about the optical axis by a motor. By moving the lens holding frame forwardly and backwardly along the optical axis direction by means of a cam slot formed at the inner circumference of the cam tube and a straight-motion guide slot provided at the guide tube, the tube collapsing operation and the zooming operation are carried out.
Furthermore, in order to prevent the cam follower from disengaging from the cam slot if an external force such as impact is applied when the lens holding frame is being protruded, the shape of the cam slot is defined by a perpendicular surface being perpendicular to the inner circumferential surface of the cam tube as well as a slant surface.
However, if the external force is large, even with the aforementioned lens barrel there is a possibility that the cam follower disengages from the cam slot. Generally, the cam tube is a molding being molded by injection molding, while the cam follower is a metal component of circular shape. Therefore, if the external force is large, the cam follower may shave the cam slot and disengage from it.
Thus, although the proof strength (proof stress) to the external force may be better in the structure in which the cam slot has a perpendicular surface as in the aforementioned lens barrel, in comparison with the structure in which the cam slot is formed only by slant surfaces as of a trapezoid shape, further improvement of the proof strength is desirable.